1. Field of the Invention
The present invention relates to an image forming apparatus capable of performing a calibration process for adjusting an image forming condition.
2. Description of the Related Art
An electrophotographic image forming apparatus such a printer, copier, facsimile machine or a complex machine of these includes an image bearing member such as a photoconductive drum and a developing unit for forming an output image (toner image) by supplying developer (toner). The outer surface of the image bearing member is uniformly charged by a charger and exposed by the irradiation of a laser light to form an electrostatic latent image.
The developing unit is arranged near the image bearing member and includes a developer bearing member (development sleeve) for bearing the developer. The developer on the developer bearing member is excited by a development bias voltage to adhere to the electrostatic latent image on the outer surface of the image bearing member. In this way, the above output image is formed on the outer surface of the image bearing member.
Generally, an image forming apparatus has image forming parameters, which are operating conditions of the image forming apparatus influential to the density setting of the output image and adopted to ensure good image quality. These image forming parameters include, for instance, exposure conditions such as the exposure power of laser light to be irradiated to the image bearing member, charging conditions of the outer surface of the image bearing member and developing conditions such as a development bias voltage of a developer bearing member and a surface speed ratio (hereinafter, “circumferential speed ratio”) between the developer bearing member and the image bearing member. These conditions change with time or according to a surrounding environment. Therefore, good image quality cannot be constantly ensured if the image forming parameters are fixed.
Because of this situation, a calibration process has been conventionally performed to properly adjust the above image forming parameters. An example of a conventional calibration process is a process for forming a solid patch image on an image bearing member, detecting the density of this image and adjusting image forming parameters such that the detected density satisfies a target density. It should be noted that the solid patch image is a toner image solid to have a fixed density in a specified area of the image bearing member.
On the other hand, Japanese Unexamined Patent Publication No. H09-50155 (D1) discloses a calibration process in the following process of (1)→(2).
(1) A solid patch image is formed, the density thereof is detected, such a circumferential speed ratio (ratio between the circumferential speed of the image bearing member and that of the development sleeve) that the detected density is a target density is estimated and the estimated circumferential speed ratio is set as an image forming parameter for an image forming process.
(2) A line test image (image in which a plurality of line images are arranged side by side) is formed at the circumferential speed ratio set in the above process (1), exposure power necessary to obtain an optimal line width is estimated based on the detected density, and the estimated exposure power is set as an image forming parameter for the image forming process.
The image density is generally detected by detecting the quantity of light irradiated to a specified area of an image and reflected thereby. The density of the solid patch image detected by this method changes to a very small extent in relation to an increase in the amount of developer supplied to the image bearing member since the reflected light quantity saturates if the amount of the developer (hereinafter, “developer amount”) supplied to the image bearing member exceeds a specified quantity by adjustments of the image forming parameters such as the development bias voltage. Thus, if the developer amount is mainly adjusted based on the detected density of only the solid patch image, the developer amount tends to be excessive to ensure the density of the solid patch image. As a result, there is a problem of degrading image quality due to image damage by the excessive supply of the developer upon developing images of characters or line drawings.
Thus, the image forming parameters need to be adjusted also in consideration of image qualities of characters, line drawings, etc. so that the developer is not excessively supplied upon developing images of characters or line drawings. However, no sufficient consideration is made on this point in the above D1 document. In other words, the exposure power is changed based on the detected density of the line test image according to the technology of the D1 document, but it is already known that a change of the exposure power is unlikely to be reflected on the developer amount. Therefore, it is difficult to solve the excessive supply of the developer by the exposure power.
The change of the exposure power also causes the following problem. For example, in a negative charging system, the width of line images (hereinafter, “line width”) increases if the exposure power is increased. Thus, image damage is more unlikely to occur upon developing images of characters and line drawings, whereby image quality is further degraded. On the other hand, the line width decreases if the exposure power is decreased, but the density of the output image is simultaneously decreased to result in light images, wherefore good images of characters and line drawings are not formed.
Here, the circumferential speed ratio exists as the image forming parameter for adjusting the supplied amount of the developer to the image bearing member. If the circumferential speed ratio is excessively large, the line width becomes excessively thick in a sub scanning direction. If the circumferential speed ratio is excessively small, an opposite situation occurs.